A longitudinal comparison of the responses of preadolescent passive smoking and nonsmoking twins and their parents is proposed which will test the following hypotheses: (1) Genetic factors account for a significant proportion of the variation in the hematologic and cardiovascular determinants of systemic oxygen delivery. (2) Adaptive responses of the oxygen delivery system differ in the same individual before and after puberty. (3) Passive smoking in children is an incremental risk factor for the development of accelerated atherosclerotic/ischemic cardiovascular disease. (4) Passive smoking in children in a contributing factor in the development of reactive airway disease. A sample of 300 preadolescent twin pairs stratified by sex, zygosity and cigarette smoke exposure, will be recruited from an established population-based twin study. A resting and exercise noninvasive evaluation of hemodynamic (resting blood pressure, heart rate, echocardiographic evaluation of cardiac output and left ventricular mass, and exercise heart rate, blood pressure and oxygen consumption), hematologic (hematocrit and blood levels of 2,3-DPG, continine, thiocyanate and erythropoietin), hemorheologic (viscosity profile) and pulmonary (spirometry and flow/volume loops) components of the oxygen transport system will be performed. Serum lipoprotein levels will be measured. Three cohorts of 11-year-old twins will initially be evaluated in a cross-sectional study and then followed longitudinally for up to 3 years. The availability of repeated measures of oxygen delivery and its determinants in twins and their parents will permit both a unique analysis of genetic and environmental factors during the process of developmental change and measurements of the risks of accelerated atherosclerotic/ischemic heart disease and of the development of reactive airway disease.